Light-Emitting Element, Light-Emitting Device, and Electronic Device

ABSTRACT

It is an object of the present invention to provide a light-emitting element with high light emission efficiency and with a long lifetime. A light-emitting device comprises a first electrode, a second electrode, a light-emitting layer, a first layer, and a second layer, wherein the first layer is provided between the light-emitting layer and the first electrode, the second layer is provided between the light-emitting layer and the second electrode, the first layer is a layer for controlling the hole transport, the second layer is a layer for controlling the electron transport, and light emission from the light-emitting layer is obtained when voltage is applied to the first electrode and the second electrode so that potential of the first electrode is higher than potential of the second electrode.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to current excitation type light-emittingelements. In addition, the present invention relates to light-emittingdevices and electronic devices which have the light-emitting elements.

2. Description of the Related Art

In recent years, research and development have been extensivelyconducted on light-emitting elements utilizing electroluminescence. In abasic structure of such a light-emitting element, a substance having alight-emitting property is interposed between a pair of electrodes. Byapplying voltage to this element, light can be emitted from thesubstance having a light-emitting property.

Since such a light-emitting element is a self-light-emitting type, ithas advantages over a liquid crystal display such as higher visibilityof pixels and unnecessity of a backlight. Accordingly, such alight-emitting element is considered suitable as a flat panel displayelement. In addition, other advantages of such a light-emitting elementare that it can be manufactured to be thin and lightweight and theresponse speed is very high.

Furthermore, since such a light-emitting element can be formed into afilm shape, surface light emission can be easily obtained by forming alarge-area element. This is a feature that is difficult to be obtainedfrom a point light source typified by a filament lamp and an LED or alinear light source typified by a fluorescent light. Therefore, thelight-emitting element has a high utility value as a plane light sourcethat can be applied to lighting or the like.

Light-emitting elements utilizing electroluminescence are classifiedbroadly according to whether they use an organic compound or aninorganic compound as a substance having a light-emitting property.

When an organic compound is used as a substance having a light-emittingproperty, electrons and holes are injected into a layer containing theorganic compound having a light-emitting property from a pair ofelectrodes by voltage application to a light-emitting element, so thatcurrent flows therethrough. Then, these carriers (electrons and holes)are recombined; thus, the organic compound having a light-emittingproperty is brought into an excited state. When the light-emittingorganic compound returns to a ground state from the excited state, itemits light. Based on this mechanism, such a light-emitting element isreferred to as a current-excitation light-emitting element.

Note that the excited state of an organic compound can be either asinglet excited state or a triplet excited state, and light emissionfrom the singlet excited state is referred to as fluorescence and lightemission from the triplet excited state is referred to asphosphorescence.

As for such a light-emitting element, there are many problems dependingon materials in improving element characteristics, and improvement inelement structure, development of materials, and the like have beenconducted to overcome the problems.

For example, in Non-Patent Document 1 (Non-Patent Document 1: TetsuoTSUTSUI and eight others, Japanese Journal of Applied Physics vol. 38,L1502 to L1504, (1999)), a hole-blocking layer is provided so that alight-emitting element using a phosphorescent material efficiently emitslight. However, as described in Non-Patent Document 1, a hole-blockinglayer has poor durability, and the light-emitting element has a veryshort lifetime. Thus, development of a light-emitting element with along lifetime has been desired.

SUMMARY OF THE INVENTION

In view of the foregoing problems, it is an object of the presentinvention to provide a light-emitting element with a long lifetime. Inaddition, it is another object of the present invention to provide alight-emitting device and an electronic device with a long lifetime.

As a result of diligent studies, the present inventors have found that alight-emitting element with a long lifetime can be obtained by providinga layer for controlling the carrier transport.

One aspect of the present invention is a light-emitting elementincluding a light-emitting layer, a first layer, and a second layerbetween a first electrode and a second electrode, wherein the firstlayer is provided between the light-emitting layer and the firstelectrode, the second layer is provided between the light-emitting layerand the second electrode, the first layer contains a first organiccompound and a second organic compound, the weight percent of the firstorganic compound is higher than the weight percent of the second organiccompound in the first layer, the first organic compound has ahole-transporting property, the second organic compound has anelectron-transporting property, the second layer contains a thirdorganic compound and a fourth organic compound, the weight percent ofthe third organic compound is higher than the weight percent of thefourth organic compound in the second layer, the third organic compoundhas an electron-transporting property, the fourth organic compound hasan electron-trapping property, and light emission from thelight-emitting layer can be obtained when voltage is applied to thefirst electrode and the second electrode so that potential of the firstelectrode is higher than potential of the second electrode.

In the above structure, the difference between the highest occupiedmolecular orbital levels of the first organic compound and the secondorganic compound is preferably less than 0.3 eV.

In the above structure, the first organic compound is preferably anaromatic amine compound and the second organic compound is preferably ametal complex.

In the above structure, P₁/P₂≧3 or P₁/P₂≦0.33 is preferably satisfiedwhere a dipole moment of the first organic compound is P₁ and a dipolemoment of the second organic compound is P₂.

In the above structure, the thickness of the first layer is preferablyfrom 5 nm to 20 nm, inclusive.

In the above structure, the first layer and the light-emitting layer arepreferably provided to be in contact with each other.

In addition, the lowest unoccupied molecular orbital level of the fourthorganic compound is preferably lower than the lowest unoccupiedmolecular orbital level of the third organic compound by 0.3 eV or more.

In addition, the third organic compound is preferably a metal complex.

In addition, the fourth organic compound is preferably a coumarinderivative or a quinacridone derivative.

In the above structure, the thickness of the second layer is preferablyfrom 5 nm to 20 nm, inclusive.

In the above structure, the second layer and the light-emitting layerare preferably provided to be in contact with each other.

The present invention includes a light-emitting device having theabove-described light-emitting element. The light-emitting device inthis specification includes an image display device, a light-emittingdevice, or a light source (including a lighting device). Further, thefollowing are also referred to as a light-emitting device: a module inwhich a connector, for example, a flexible printed circuit (FPC), a tapeautomated bonding (TAB) tape, or a tape carrier package (TCP) isattached to a panel provided with a light-emitting element; a moduleprovided with a printed wiring board at the end of the TAB tape or theTCP; and a module in which an integrated circuit (IC) is directlymounted to a light-emitting element by a chip on glass (COG) method.

Further, an electronic device using the light-emitting element of thepresent invention in its display portion is also included in the presentinvention. Accordingly, an electronic device of the present inventionincludes a display portion which is provided with the above-describedlight-emitting element and a control means to control light emission ofthe light-emitting element.

In the light-emitting element of the present invention, a layer forcontrolling the carrier transport is provided; therefore, alight-emitting element with a long lifetime can be obtained.

Further, the light-emitting device and the electronic device with a longlifetime can be obtained by applying a light-emitting element of thepresent invention to a light-emitting device and an electronic device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1D each illustrate a light-emitting element of the presentinvention.

FIGS. 2A to 2D each illustrate a light-emitting element of the presentinvention.

FIGS. 3A to 3C each illustrate a light-emitting element of the presentinvention.

FIG. 4 illustrates a light-emitting element of the present invention.

FIG. 5 illustrates a light-emitting element of the present invention.

FIG. 6 illustrates a light-emitting element of the present invention.

FIGS. 7A and 7B illustrate a light-emitting device of the presentinvention.

FIGS. 8A and 8B illustrate a light-emitting device of the presentinvention.

FIGS. 9A to 9D each illustrate an electronic device of the presentinvention.

FIG. 10 illustrates an electronic device of the present invention.

FIG. 11 illustrates an electronic device of the present invention.

FIG. 12 illustrates an electronic device of the present invention.

FIG. 13 illustrates a lighting device of the present invention.

FIG. 14 illustrates a lighting device of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The embodiment modes of the present invention are hereinafter describedin detail with reference to the drawings. Note that the presentinvention is not limited to the following description and it will bereadily appreciated by those who skilled in the art that modes anddetails can be modified in various ways without departing from thespirit and the scope of the present invention. Accordingly, the presentinvention should not be construed as being limited to the description ofthe embodiment modes to be given below.

Embodiment Mode 1

One mode of a light-emitting element according to the present inventionis hereinafter described with reference to FIGS. 1A to 1D. Alight-emitting element according to the present invention has a layerfor controlling the hole transport and a layer for controlling theelectron transport.

A light-emitting element of the present invention has a plurality oflayers between a pair of electrodes. The plurality of layers are acombination of layers formed of a material having a highcarrier-injecting property and a material having a highcarrier-transporting property. Those layers are stacked so that alight-emitting region is formed in a region away from the electrodes,that is, carriers are recombined in an area away from the electrodes.

In this embodiment mode, a light-emitting element has a first electrode202, a second electrode 204, and an EL layer 203 provided between thefirst electrode 202 and the second electrode 204. Note that thisembodiment mode is described below assuming that the first electrode 202serves as an anode and the second electrode 204 serves as a cathode.That is, in the following description, it is assumed that when voltageis applied to the first electrode 202 and the second electrode 204 sothat potential of the first electrode 202 is higher than potential ofthe second electrode 204, light is emitted.

A substrate 201 is used as a support of the light-emitting element. Forthe substrate 201, glass, plastic, or the like can be used, for example.Note that other materials may also be used as long as they serve as asupport in a manufacturing process of the light-emitting element.

As for the first electrode 202, a metal, an alloy, a conductivecompound, a mixture thereof, or the like having a high work function(specifically, preferably 4.0 eV or higher) is preferably used. Forexample, indium tin oxide (ITO), indium tin oxide containing silicon orsilicon oxide, indium zinc oxide (IZO), indium oxide containing tungstenoxide and zinc oxide (IWZO), and the like can be given. A film of such aconductive metal oxide is generally formed by sputtering, but may beformed by an inkjet method, a spin coating method, or the like byapplication of a sol-gel method or the like. For example, a film ofindium zinc oxide (IZO) can be formed by a sputtering method using atarget in which zinc oxide is added into indium oxide at 1 to 20 wt %.Further, a film of indium oxide containing tungsten oxide and zinc oxide(IWZO) can be formed by a sputtering method using a target in whichtungsten oxide and zinc oxide are included in indium oxide at 0.5 to 5wt % and at 0.1 to 1 wt %, respectively. Besides, gold (Au), platinum(Pt), nickel (Ni), tungsten (W), chromium (Cr), molybdenum (Mo), iron(Fe), cobalt (Co), copper (Cu), palladium (Pd), titanium (Ti), nitrideof a metal material (e.g., titanium nitride), and the like can be given.

In a case where a layer containing a composite material described belowis used as a layer which is in contact with the first electrode, any ofvarious metals, alloys, electrically conductive compounds, or a mixturethereof can be used for the first electrode regardless of the workfunction. For example, aluminum (Al), silver (Ag), an alloy containingaluminum (AlSi), or the like can be used. Besides, any of the followingmaterials with a low work function can be used for the first electrode:elements belonging to Group 1 and Group 2 of the periodic table, thatis, alkali metals such as lithium (Li) and cesium (Cs) and alkalineearth metals such as magnesium (Mg), calcium (Ca), and strontium (Sr);alloys thereof (e.g., MgAg and AlLi); rare earth metals such as europium(Eu) and ytterbium (Yb), and alloys thereof; and the like. A film of analkali metal, an alkaline earth metal, or an alloy thereof can be formedby a vacuum evaporation method. In addition, a film of an alloyincluding an alkali metal or an alkaline earth metal can be formed by asputtering method. Further, a film can be formed using a silver paste orthe like by an inkjet method or the like.

The EL layer 203 in this embodiment mode includes a hole-injecting layer211, a hole-transporting layer 212, a layer 213 for controlling the holetransport, a light-emitting layer 214, a layer 215 for controlling theelectron transport, an electron-transporting layer 216, and anelectron-injecting layer 217. Note that as long as the EL layer 203includes a layer for controlling the carrier transport and alight-emitting layer in this embodiment mode, a stacked structure ofother layers is not specifically limited. That is, there is noparticular limitation on the stacked structure of the EL layer 203, anda layer for controlling the carrier transport and a light-emitting layerin this embodiment mode may be combined with a layer formed of asubstance having a high electron-transporting property, a substancehaving a high hole-transporting property, a substance having a highelectron-injecting property, a substance having a high hole-injectingproperty, a bipolar substance (a substance having highelectron-transporting and hole-transporting properties), or the like.For example, the EL layer 203 can be formed by an appropriatecombination of a hole-injecting layer, a hole-transporting layer, alight-emitting layer, an electron-transporting layer, anelectron-injecting layer, and the like. Specific materials for each ofthe layers are given below.

The hole-injecting layer 211 is a layer containing a substance having ahigh hole-injecting property. As a substance having a highhole-injecting property, molybdenum oxide, vanadium oxide, rutheniumoxide, tungsten oxide, manganese oxide, or the like can be used. Asanother low molecular organic compound, a phthalocyanine-based compoundsuch as phthalocyanine (abbreviation: H₂Pc), copper(II)phthalocyanine(abbreviation: CuPc), or vanadyl phthalocyanine (VOPc); an aromaticamine compound such as 4,4′,4″-tris(N,N-diphenylamino)triphenylamine(abbreviation: TDATA),4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine(abbreviation: MTDATA),4,4′-bis[N-(4-diphenylaminophenyl)-N-phenylamino]biphenyl (abbreviation:DPAB),4,4′-bis(N-{4-[N-(3-methylphenyl)-N′-phenylamino]phenyl}-N-phenylamino)biphenyl(abbreviation: DNTPD),1,3,5-tris[N-(4-diphenylaminophenyl)-N-phenylamino]benzene(abbreviation: DPA3B),3-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA1),3,6-bis[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA2), or3-[N-(1-naphthyl)-N-(9-phenylcarbazol-3-yl)amino]-9-phenylcarbazole(abbreviation: PCzPCN1); or the like can be given.

Alternatively, for the hole-injecting layer 211, a composite material inwhich a substance having an acceptor property is mixed into a substancehaving a high hole-transporting property can be used. Note that by usinga material in which a substance having an acceptor property is mixedinto a substance having a high hole-transporting property, a materialfor forming the electrode can be selected regardless of its workfunction. In other words, besides a material with a high work function,a material with a low work function can be used as the first electrode202. A composite material of those substances can be formed byco-evaporation of a substance having a high hole-transporting propertyand a substance having an acceptor property.

Note that in this specification, “composition” refers to not only astate where two materials are simply mixed but also a state where aplurality of materials are mixed and charge is given and receivedbetween the materials.

As an organic compound which is used for a composite material, any ofvarious compounds such as an aromatic amine compound, a carbazolederivative, aromatic hydrocarbon, or a high molecular compound (anoligomer, a dendrimer, a polymer, or the like) can be used. Note that anorganic compound which is used for the composite material is preferablyan organic compound having a high hole-transporting property.Specifically, a substance having a hole mobility of 10⁻⁶ cm²/Vs orhigher is preferably used. However, another substance may be used aslong as the hole-transporting property thereof is higher than theelectron-transporting property. Examples of an organic compound that canbe used for the composite material are specifically listed below.

The organic compound which can be used for the composite material is,for example, an aromatic amine compound such as MTDATA, TDATA, DPAB,DNTPD, DPA3B, PCzPCA1 PCzPCA2, PCzPCN1,4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB orα-NPD), orN,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine(abbreviation: TPD); a carbazole derivative such as4,4′-di(N-carbazolyl)biphenyl (abbreviation: CBP),1,3,5-tris[4-(N-carbazolyl)phenyl]benzene (abbreviation: TCPB),9-[4-(N-carbazolyl)]phenyl-10-phenylanthracene (abbreviation: CzPA), or1,4-bis[4-(N-carbazolyl)phenyl]-2,3,5,6-tetraphenylbenzene; or anaromatic hydrocarbon compound such as2-tert-butyl-9,10-di(2-naphthyl)anthracene (abbreviation: t-BuDNA),2-tert-butyl-9,10-di(1-naphthyl)anthracene,9,10-bis(3,5diphenylphenyl)anthracene (abbreviation: DPPA),2-tert-butyl-9,10-bis(4-phenylphenyl)anthracene (abbreviation: t-BuDBA),9,10-di(2-naphthyl)anthracene (abbreviation: DNA),9,10-diphenylanthracene (abbreviation: DPAnth), 2-tert-butylanthracene(abbreviation: t-BuAnth), 9,10-bis(4-methyl-1-naphthyl)anthracene(abbreviation: DMNA),9,10-bis[2-(1-naphthyl)phenyl]-2-tert-butylanthracene,9,10-bis[2-(1-naphthyl)phenyl]anthracene,2,3,6,7-tetramethyl-9,10-di(1-naphthyl)anthracene,2,3,6,7-tetramethyl-9,10-di(2-naphthyl)anthracene, 9,9′-bianthryl,10,10′-diphenyl-9,9′-bianthryl,10,10′-bis(2-phenylphenyl)-9,9′-bianthryl,10,10′-bis[(2,3,4,5,6-pentaphenyl)phenyl]-9,9′-bianthryl, anthracene,tetracene, rubrene, perylene, 2,5,8,11-tetra(tert-butyl)perylene,pentacene, coronene, 4,4′-bis(2,2-diphenylvinyl)biphenyl (abbreviation:DPVBi), or 9,10-bis[4-(2,2-diphenylvinyl)phenyl]anthracene(abbreviation: DPVPA).

As the substance having an acceptor property, an organic compound suchas 7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane (abbreviation:F4-TCNQ) or chloranil; or a transition metal oxide can be given. Inaddition, oxide of metals that belong to Group 4 to Group 8 of theperiodic table can be given as the substance having an acceptorproperty. Specifically, vanadium oxide, niobium oxide, tantalum oxide,chromium oxide, molybdenum oxide, tungsten oxide, manganese oxide, andrhenium oxide are preferable because of their high electron-acceptingproperty. Among these, molybdenum oxide is preferable because it can beeasily handled because of its stableness in the air and low hygroscopicproperty.

Further, for the hole-injecting layer 211, a high molecular compound (anoligomer, a dendrimer, a polymer, or the like) can be used. For example,a high molecular compound such as poly(N-vinylcarbazole) (abbreviation:PVK), poly(4-vinyltriphenylamine) (abbreviation: PVTPA),poly[N-(4-{N′-[4-(4-diphenylamino)phenyl]phenyl-N′-phenylamino}phenyl)methacrylamide](abbreviation: PTPDMA), orpoly[N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)benzidine (abbreviation:Poly-TPD) can be given. In addition, a high molecular compound to whichacid, for example, poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonicacid) (PEDOT/PSS), or polyaniline/poly(styrenesulfonic acid) (PAni/PSS)is added can be used.

Further, a composite material formed by using the above-mentioned highmolecular compound such as PVK, PVTPA, PTPDMA, or Poly-TPD and theabove-mentioned substance having an acceptor property can be used forthe hole-injecting layer 211.

The hole-transporting layer 212 is a layer containing a substance havinga high hole-transporting property. As a substance having a highhole-transporting property, an aromatic amine compound such as NPB (orα-NPD), TPD,4,4′-bis[N-(9,9-dimethylfluoren-2-yl)-N-phenylamino]biphenyl(abbreviation: DFLDPBi),4,4′-bis[N-(spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl(abbreviation: BSPB), or the like, which is a low molecular organiccompound, can be used. Most of the substances mentioned here have a holemobility of 10⁻⁶ cm²/Vs or higher. Note that another substance may beused as long as the hole-transporting property thereof is higher thanthe electron-transporting property. Note that the layer containing asubstance having a high hole-transporting property is not limited to asingle layer, and may be two or more stacked layers containing any ofthe above-mentioned substances.

Further, for the hole-transporting layer 212, a high molecular compoundsuch as PVK, PVTPA, PTPDMA, or Poly-TPD can be used.

The layer 213 for controlling the hole transport contains a firstorganic compound and a second organic compound and the weight percent ofthe first organic compound is higher than that of the second organiccompound. The layer 213 for controlling the hole transport is preferablyprovided between the light-emitting layer 214 and the first electrode202.

The layer 213 for controlling the hole transport in this embodiment modecontains the first organic compound and the second organic compound, andthe first organic compound and the second organic compound transportdifferent kinds of carriers.

In a case where the layer for controlling the hole transport is providedbetween the light-emitting layer and the second electrode serving as acathode, the first organic compound is preferably an organic compoundhaving a hole-transporting property, and the second organic compound ispreferably an organic compound having an electron-transporting property.That is, the first organic compound is preferably a substance whoseelectron-transporting property is higher than the hole-transportingproperty, while the second organic compound is preferably a substancewhose hole-transporting property is higher than theelectron-transporting property. In addition, the difference between thehighest occupied molecular orbital (HOMO) level of the first organiccompound and that of the second organic compound is preferably less than0.3 eV, and more preferably 0.2 eV or less. That is, it is preferablethat, in thermodynamic terms, holes, which are carriers, can be easilytransported between the first organic compound and the second organiccompound.

FIG. 4 illustrates a conceptual view of a layer for controlling thecarrier transport in this embodiment mode. In FIG. 4, since a firstorganic compound 221 has a hole-transporting property, holes are easilyinjected thereinto and transported to neighboring first organiccompound. That is, the rate at which holes are injected into the firstorganic compound and the rate (ν) at which the holes are released fromthe first organic compound are high.

Meanwhile, in thermodynamic terms, there is a possibility that holes areinjected into a second organic compound 222 which is an organic compoundhaving an electron-transporting property because the HOMO level of thesecond organic compound 222 is close to that of the first organiccompound 221. However, the rate (ν₁) at which holes are injected fromthe first organic compound 221, which is an organic compound having ahole-transporting property, into the second organic compound 222, whichis an organic compound having an electron-transporting property, or therate (ν₂), at which holes are injected from the second organic compound222 into the first organic compound 221, is lower than the rate (ν) atwhich holes are injected from the first organic compound 221 intoanother first organic compound 221.

Since the second organic compound is contained, the hole-transportingrate of the entire layer is lower than that of a layer containing onlythe first organic compound 221. That is, by adding the second organiccompound, the carrier transport can be controlled. Further, bycontrolling the concentration of the second organic compound, thecarrier transporting rate can be controlled.

As described above, the first organic compound is preferably an organiccompound having a hole-transporting property in this embodiment mode.Specifically, a condensed aromatic hydrocarbon such as9,10-diphenylanthracene (abbreviation: DPAnth) or6,12-dimethoxy-5,11-diphenylchrysene, an aromatic amine compound such asN,N-dipheyl-9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazol-3-amine(abbreviation: CzA1PA), 4-(10-phenyl-9-anthryl)triphenylamine(abbreviation: DPhPA),N,9-diphenyl-N-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazol-3-amine(abbreviation: PCAPA),N,9-diphenyl-N-{4-[4-(10-phenyl-9-anthryl)phenyl]phenyl}-9H-carbazol-3-amine(abbreviation: PCAPBA),N-(9,10-diphenyl-2-anthryl)-N,9-diphenyl-9H-carbazol-3-amine,(abbreviation: 2PCAPA), NPB (or α-NPD), TPD, DFLDPBi, or BSPB, acompound including an amino group such as Coumarin 7, or Coumarin 30,and the like can be used. Further, a high molecular compound such asPVK, PVTPA, PTPDMA, or Poly-TPD can be used.

As the second organic compound 222, an organic compound having anelectron-transporting property is preferably used. Specifically, a metalcomplex such as tris(8-quinolinolato)aluminum(III) (abbreviation: Alq),tris(4-methyl-8-quinolinolato)aluminum(III) (abbreviation: Almq₃),bis(10-hydroxybenzo[h]quinolinato)beryllium(II) (abbreviation: BeBq₂),bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum(III)(abbreviation: BAlq), bis(8-quinolinolato)zinc(II) (abbreviation: Znq),bis[2-(2-benzoxazolyl)phenolato]zinc(II) (abbreviation: ZnPBO), orbis[2-(2-benzothiazolyl)phenolato]zinc(II) (abbreviation: Zn(BTZ)₂), canbe used. Further, as an alternative to such a metal complex, aheterocyclic compound such as2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation:PBD), 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene(abbreviation: OXD-7),3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole(abbreviation: TAZ01),2,2′,2″-(1,3,5-benzenetriyl)tris(1-phenyl-1H-benzimidazole)(abbreviation: TPBI), bathophenanthroline (abbreviation: BPhen), orbathocuproine (abbreviation: BCP) can be used. Further, a condensedaromatic compound such as 9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole(abbreviation: CzPA),3,6-diphenyl-9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole(abbreviation: DPCzPA), 9,10-bis(3,5diphenylphenyl)anthracene(abbreviation: DPPA), 9,10-di(2-naphthyl)anthracene (abbreviation: DNA),2-tert-butyl-9,10-di(2-naphthyl)anthracene (abbreviation: t-BuDNA),9,9′-bianthryl (abbreviation: BANT),9,9′-(stilbene-3,3′-diyl)diphenanthrene (abbreviation: DPNS),9,9′-(stilbene-4,4′-diyl)diphenanthrene (abbreviation: DPNS2), or3,3′,3″-(benzene-1,3,5-triyl)tripyrene (abbreviation: TPB3) can be used.Further, a high molecular compound such aspoly[(9,9-dihexylfluorene-2,7-diyl)-co-(pyridine-3,5-diyl)](abbreviation: PF-Py), orpoly[(9,9-dioctyllfluorene-2,7-diyl)-co-(2,2′-bipyridine-6,6′-diyl)](abbreviation: PF-BPy) can be used.

By the above combination, the hole transport from the first organiccompound to the second organic compound or from the second organiccompound to the first organic compound is suppressed, whereby thehole-transporting rate in the layer for controlling the carriertransport can be suppressed. Further, the layer for controlling thecarrier transport has a structure in which the second organic compoundis dispersed into the first organic compound; therefore, crystallizationor aggregation is hardly caused with time. Accordingly, theabove-described effect of suppressing the hole transport is hardlychanged with time, and as a result, the carrier balance hardly changeswith time. This leads to improvement in lifetime, in other words,improvement in reliability of the light-emitting element.

Note that among the above-described combinations, an aromatic aminecompound and a metal complex are preferably combined as the firstorganic compound and the second organic compound, respectively. Anaromatic amine compound has a high hole-transporting property and asmall dipole moment, whereas a metal complex has a highelectron-transporting property and a comparatively large dipole moment.In such a manner, by combination of substances dipole moments of whichare largely different from each other, the above-described effect ofsuppressing the hole transport becomes more significant.

Specifically, a combination which satisfies P₁/P₂≧3 or P₁/P₂≦0.33 wherethe dipole moment of the first organic compound is P₁ and the dipolemoment of the second organic compound is P₂ is preferable. For example,the dipole moment of NPB that is one of aromatic amine compounds is 0.86debye, and the dipole moment of Alq that is a metal complex is 9.40debye. Accordingly, as in this embodiment mode, when an organic compoundhaving a hole-transporting property like an aromatic amine compound isused as the first organic compound and an organic compound having anelectron-transporting property like a metal complex is used as thesecond organic compound, P₁/P₂≦0.33 is preferably satisfied.

In addition, it is preferable that the emission color of the secondorganic compound contained in the layer 213 for controlling the holetransport and the emission color of the substance having a highlight-emitting property contained in the light-emitting layer 214 besimilar colors. Specifically, it is preferable that the differencebetween the wavelength of the highest peak of the emission spectrum ofthe second organic compound and the wavelength of the highest peak ofthe emission spectrum of the substance having a high light-emittingproperty be 30 nm or less. When the difference between the wavelengthsof the highest peaks is 30 nm or less, the emission colors of the secondorganic compound and the substance having a high light-emitting propertycan be similar colors. Accordingly, even in the case where the secondorganic compound emits light due to change in voltage or the like,change in emission color can be suppressed. Note that the second organiccompound does not always need to emit light.

In addition, the thickness of the layer 213 for controlling the holetransport is preferably from 5 nm to 20 nm, inclusive. When thethickness of the layer 213 for controlling the hole transport is toolarge, the carrier transporting rate becomes too slow, which couldresult in high driving voltage, in addition, the emission intensity ofthe layer 213 for controlling the hole transport may increase. When thethickness of the layer 213 for controlling the hole transport is toosmall, on the other hand, it is impossible to implement the function ofcontrolling the carrier transport. Therefore, the thickness ispreferably from 5 nm to 20 nm, inclusive.

In a conventional light-emitting element where a layer for controllingthe hole transport is not provided, holes injected from the firstelectrode pass through a hole-injecting layer and a hole-transportinglayer to be injected into a light-emitting layer. If the light-emittinglayer has a hole-transporting property, that is, if the material whichhas the highest weight percent in the light-emitting layer has ahole-transporting property, holes injected into the light-emitting layertransfer through the light-emitting layer, and may reach anelectron-transporting layer. When holes reach the electron-transportinglayer, materials contained in the electron-transporting layer aredegraded, leading deterioration of the light-emitting element.

However, by providing the layer 213 for controlling the hole transportdescribed in this embodiment mode, it is possible to suppress the holespenetrating the light-emitting layer 214 and reaching theelectron-transporting layer 216. Therefore, deterioration of theelectron-transporting layer 216, which is caused by holes reaching theelectron-transporting layer 216, can be suppressed. Accordingly,deterioration of the light-emitting element can be prevented, and thelight-emitting element with a long lifetime can be obtained.

On the other hand, in a conventional element where the layer forcontrolling the hole transport is not provided, most of the holesinjected from the first electrode are injected into the light-emittinglayer without the transport being controlled. If the light-emittinglayer has an electron-transporting property, that is, if a materialwhich has the highest weight percent in the light-emitting layer has anelectron-transporting property, a light-emitting region is formed in thevicinity of the interface between the light-emitting layer and thehole-transporting layer. In addition, there is a possibility thatcations are generated by excessive holes in the vicinity of theinterface between the light-emitting layer and the hole-transportinglayer. Since a cation serves as a quencher, light emission efficiencydecreases due to cations generated in the vicinity of the light-emittingregion.

However, by providing the layer 213 for controlling the hole transportdescribed in this embodiment mode, formation of cations generated byexcessive holes in the light-emitting layer 214 and in the vicinity ofthe light-emitting layer 214 can be suppressed, and decrease in lightemission efficiency can be suppressed. Accordingly, a light-emittingelement with high light emission efficiency can be obtained.

As described above, by controlling the hole transport, the carrierbalance is improved. As a result, the recombination probability of holesand electrons is improved and high light emission efficiency can beobtained. Note that as described in this embodiment mode, a structure inwhich the layer for controlling the hole transport is provided betweenthe light-emitting layer and the first electrode serving as an anode isparticularly effective for a light-emitting element having excessiveholes. This is because by providing the layer for controlling the holetransport in the light-emitting element having excessive holes, thetransport of excessive holes can be suppressed and controlled so thatthe balance of holes and electrons can be achieved.

The light-emitting layer 214 is a layer containing a substance having ahigh light-emitting property, and various materials can be used for thelight-emitting layer 214. For example, as a substance having a highlight-emitting property, a fluorescent compound which emits fluorescenceor a phosphorescent compound which emits phosphorescence can be used.

Examples of a phosphorescent compound which is used for thelight-emitting layer are given below. As a material for bluish lightemission,bis[2-(4′,6′-difluorophenyl)pyridinato-N,C²′]iridium(III)tetrakis(1-pyrazolyl)borate(abbreviation: FIr6),bis[2-(4′,6′-difluorophenyl)pyridinato-N,C²′]iridium(III)picolinate(abbreviation: FIrpic),bis[2-(3′,5′bistrifluoromethylphenyl)pyridinato-N,C²′]iridium(III)picolinate(abbreviation: Ir(CF₃ppy)₂(pic)),bis[2-(4′,6′-difluorophenyl)pyridinato-N,C²′]iridium(III)acetylacetonate(abbreviation: FIr(acac)) or the like can be given. As a material forgreenish light emission, tris(2-phenylpyridinato-N,C²′)iridium(III)(abbreviation: Ir(Ppy)₃),bis[2-phenylpyridinato-N,C²′]iridium(III)acetylacetonate (abbreviation:Ir(ppy)₂(acac)),bis(1,2-diphenyl-1H-benzimidazolato)iridium(III)acetylacetonate(abbreviation: Ir(pbi)₂(acac)),bis(benzo[h]quinolinato)iridium(III)acetylacetonate (abbreviation:Ir(bzq)₂(acac)) or the like can be given. As a material for yellowishlight emission,bis(2,4-diphenyl-1,3-oxazolato-N,C²′)iridium(III)acetylacetonate(abbreviation: Ir(dpo)₂(acac)),bis[2-(4′-perfluorophenylphenyl)pyridinato]iridium(III)acetylacetonate(abbreviation: Ir(p-PF-ph)₂(acac)),bis(2-phenylbenzothiazolato-N,C²′)iridium(III)acetylacetonate(abbreviation: Ir(bt)₂(acac)) or the like can be given. As a materialfor orangish light emission, tris(2-phenylquinolinato-N,C²′)iridium(III)(abbreviation: Ir(Pq)₃),bis(2-phenylquinolinato-N,C²′)iridium(III)acetylacetonate (abbreviation:Ir(pq)₂(acac)) or the like can be given. As a material for reddish lightemission, organometallic complex such asbis[2-(2′-benzo[4,5-α]thienyl)pyridinato-N,C³′]iridium(III)acetylacetonate(abbreviation: Ir(btp)₂(acac)),bis(1-phenylisoquinolinato-N,C²′)iridium(III)acetylacetonate(abbreviation: Ir(piq)₂(acac)),(acetylacetonato)bis[2,3-bis(4-fluorophenyl)quinoxalinato]iridium(III)(abbreviation: Ir(Fdpq)₂(acac)), or(2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrinato)platinum(II)(abbreviation: PtOEP) can be given. In addition, a rare earth metalcomplex such as tris(acetylacetonato)(monophenanthroline)terbium(III)(abbreviation: Tb(acac)₃(Phen)),tris(1,3-diphenyl-1,3-propanedionato)(monophenanthroline)europium(III)(abbreviation: Eu(DBM)₃(Phen)), ortris[1-(2-thenoyl)-3,3,3-trifluoroacetonato](monophenanthroline)europium(III)(abbreviation: Eu(TTA)₃(Phen)) performs light emission (electrontransition between different multiplicities) from a rare earth metalion; therefore, such a rare earth metal complex can be used as aphosphorescent compound.

Examples of a fluorescent compound which is used for the light-emittinglayer are given below. As a material for bluish light emission,N,N′-bis[4-(9H-carbazol-9-yl)phenyl]-N,N′-diphenylstilbene-4,4′-diamine(abbreviation: YGA2S),4-(9H-carbazol-9-yl)-4′-(10-phenyl-9-anthryl)triphenylamine(abbreviation: YGAPA), or the like can be given. As a material forgreenish light emission,N-(9,10-diphenyl-2-anthryl)-N,9-diphenyl-9H-carbazol-3-amine(abbreviation, 2PCAPA),N-[9,10-bis(1,1′-biphenyl-2-yl)-2-anthryl]-N,9-diphenyl-9H-carbazol-3-amine(abbreviation: 2PCABPhA),N-(9,10-diphenyl-2-anthryl)-N,N′,N′-triphenyl-1,4-phenylenediamine(abbreviation: 2DPAPA),N-[9,10-bis(1,1′-biphenyl-2-yl)-2-anthryl]-N,N′,N′-triphenyl-1,4-phenylenediamine(abbreviation: 2DPABPhA),9,10-di(2-biphenylyl)-2-{N-[4-(9H-carbazol-9-yl)phenyl]-N-phenylamino}anthracene(abbreviation: 2YGABPhA), N,N,9-triphenylanthracen-9-amine(abbreviation: DPhAPhA), or the like can be given. As a material foryellowish light emission, rubrene,5,12-bis(1,1′-biphenyl-4-yl)-6,11-diphenyltetracene (abbreviation: BPT),or the like can be given. As a material for reddish light emission,N,N,N′,N′-tetrakis(4-methylphenyl)tetracene-5,11-diamine (abbreviation:p-mPhTD),7,13-diphenyl-N,N,N′,N′-tetrakis(4-methylphenyl)acenaphtho[1,2-α]fluoranthene-3,10-diamine(abbreviation: p-mPhAFD), or the like can be given.

Note that the light-emitting layer may have a structure in which any ofthe above substances having a high light-emitting property (a sixthorganic compound) is dispersed into another substance (a fifth organiccompound). As the fifth organic compound in which the sixth organiccompound is dispersed, various kinds of materials can be used, and it ispreferable to use the fifth organic compound whose lowest unoccupiedmolecular orbital level (LUMO level) is higher than that of the sixthorganic compound and whose highest occupied molecular orbital level(HOMO level) is lower than that of the sixth organic compound.

As the substance in which the substance having a light-emitting propertyis dispersed, specifically, a metal complex such astris(8-quinolinolato)aluminum(III) (abbreviation: Alq),tris(4-methyl-8-quinolinolato)aluminum(III) (abbreviation: Almq₃),bis(10-hydroxybenzo[h]quinolinato)beryllium(II) (abbreviation: BeBq₂),bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum(III)(abbreviation: BAlq), bis(8-quinolinolato)zinc(II) (abbreviation: Znq),bis[2-(2-benzoxazolyl)phenolato]zinc(II) (abbreviation: ZnPBO), orbis[2-(2-benzothiazolyl)phenolato]zinc(II) (abbreviation: Zn(BTZ)₂); aheterocyclic compound such as2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation:PBD), 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene(abbreviation: OXD-7),3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole(abbreviation: TAZ01),2,2′,2″-(1,3,5-benzenetriyl)tris(1-phenyl-1H-benzimidazole)(abbreviation: TPBI), bathophenanthroline (abbreviation: BPhen), orbathocuproine (BCP); a condensed aromatic compound such as9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation: CzPA),3,6-diphenyl-9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole(abbreviation: DPCzPA), 9,10-bis(3,5diphenylphenyl)anthracene(abbreviation: DPPA), 9,10-di(2-naphthyl)anthracene (abbreviation: DNA),2-tert-butyl-9,10-di(2-naphthyl)anthracene (abbreviation: t-BuDNA),9,9′-bianthryl (abbreviation: BANT),9,9′-(stilbene-3,3′-diyl)diphenanthrene (abbreviation: DPNS),9,9′-(stilbene-4,4′-diyl)diphenanthrene (abbreviation: DPNS2),3,3′,3″-(benzene-1,3,5-triyl)tripyrene (abbreviation: TPB3),9,10-diphenylanthracene (abbreviation: DPAnth), or6,12-dimethoxy-5,11-diphenylchrysene; an aromatic amine compound such asN,N-dipheyl-9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazol-3-amine(abbreviation: CzA1PA), 4-(10-phenyl-9-anthryl)triphenylamine(abbreviation: DPhPA),N,9-diphenyl-N-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazol-3-amine(abbreviation: PCAPA),N,9-diphenyl-N-{4-[4-(10-phenyl-9-anthryl)phenyl]phenyl}-9H-carbazol-3-amine(abbreviation: PCAPBA),N-(9,10-diphenyl-2-anthryl)-N,9-diphenyl-9H-carbazol-3-amine(abbreviation: 2PCAPA), NPB (or α-NPD), TPD, DFLDPBi, or BSPB; or thelike can be used.

As the substance in which the substance having a light-emitting propertyis dispersed, a plurality of kinds of substances can be used. Forexample, in order to suppress crystallization, a substance forsuppressing crystallization, such as rubrene or the like may be furtheradded. Furthermore, in order to efficiently transfer energy to thesubstance having a light-emitting property, NPB, Alq, or the like may befurther added.

When a structure in which the substance having a high light-emittingproperty is dispersed into another substance is employed,crystallization of the light-emitting layer 214 can be suppressed.Further, concentration quenching due to high concentration of thesubstance having a high light-emitting property can be suppressed.

Note that for the light-emitting layer 214, a high molecular compoundcan be used. Specifically, as a material for bluish light emission,poly(9,9-dioctylfluorene-2,7-diyl) (abbreviation: POF),poly[(9,9-dioctylfluorene-2,7-diyl-co-(2,5-dimethoxybenzene-1,4-diyl)](abbreviation: PF-DMOP),poly{(9,9-dioctylfluorene-2,7-diyl)-co-[N,N′-di-p-butylphenyl)-1,4-diaminobenzene]}(abbreviation: TAB-PFH), or the like can be given. As a material forgreenish light emission, poly(p-phenylenevinylene) (abbreviation: PPV),poly[(9,9-dihexylfluorene-2,7-diyl)-alt-co-(benzo[2,1,3]thiadiazole-4,7-diyl)](abbreviation: PFBT),poly[(9,9-dioctyl-2,7-divinylenefluorenylene)-alt-co-(2-methoxy-5-(2-ethylhexyloxy)-1,4,4-phenylene)],or the like can be given. As a material for orangish to reddish lightemission, poly[2-methoxy-5-(2′-ethylhexoxy)-1,4-phenylenevinylene](abbreviation: MEH-PPV), poly(3-butylthiophene-2,5-diyl) (abbreviation:R4-PAT),poly{[9,9-dihexyl-2,7-bis(1-cyanovinylene)fluorenylene]-alt-co-[2,5-bis(N,N′-diphenylamino)-1,4-phenylene]},poly{[2-methoxy-5-(2-ethylhexyloxy)-1,4-bis(1-cyanovinylenephenylene)]-alt-co-[2,5-bis(N,N′-diphenylamino)-1,4-phenylene]}(abbreviation: CN-PPV-DPD), or the like can be given.

The layer 215 for controlling the electron transport which is describedin this embodiment mode contains a third organic compound and a fourthorganic compound and the weight percent of the third organic compound ishigher than that of the fourth organic compound. In addition, the fourthorganic compound is dispersed into the third organic compound. The layer215 for controlling the electron transport is preferably providedbetween the light-emitting layer 214 and the second electrode 204.

In the case of providing the layer for controlling the electrontransport between the light-emitting layer and the second electrodeserving as a cathode, the third organic compound is preferably anorganic compound having an electron-transporting property. That is, thethird organic compound is preferably a substance whose hole-transportingproperty is higher than the electron-transporting property.

The fourth organic compound is preferably an organic compound having afunction of trapping electrons. That is, the fourth organic compound ispreferably an organic compound whose lowest unoccupied molecular orbitallevel (LUMO level) is lower than that of the third organic compound by0.3 eV or more.

Since the fourth organic compound is contained, theelectron-transporting rate of the entire layer is lower than that of alayer containing only the third organic compound. That is, by adding thefourth organic compound, the carrier transport can be controlled.Further, by control of the concentration of the fourth organic compound,the carrier transporting rate can be controlled. Specifically, theconcentration of the fourth organic compound is preferably from 0.1 wt %to 5 wt % or from 0.1 mol % to 5 mol %.

FIG. 5 exemplarily illustrates a band diagram of a light-emittingelement of the present invention in FIG. 1A. In FIG. 5, holes injectedfrom the first electrode 202 pass through the hole-injecting layer 211,the hole-transporting layer 212, and the layer 213 for controlling thehole transport, and are injected into the light-emitting layer 214. Onthe other hand, electrons injected from the second electrode 204 passthrough the electron-injecting layer 217, the electron-transportinglayer 216, and are injected into the layer 215 for controlling theelectron transport. The transport of the electrons injected into thelayer 215 for controlling the electron transport is retarded by thefourth organic compound having the function of trapping electrons. Theelectrons of which transport is retarded are injected into thelight-emitting layer 214, and then recombined with holes to emit light.

In a conventional element where a layer for controlling the electrontransport is not provided, electrons injected from the second electrodepass through an electron-injecting layer and an electron-transportinglayer to be injected into a light-emitting layer. If the light-emittinglayer has an electron-transporting property, that is, if the materialwhich has the highest weight percent in the light-emitting layer has anelectron-transporting property, electrons injected into thelight-emitting layer transfer through the light-emitting layer, and mayreach a hole-transporting layer. When electrons reach thehole-transporting layer, materials contained in the hole-transportinglayer are degraded, leading deterioration of the light-emitting element.

However, by providing the layer 215 for controlling the electrontransport described in this embodiment mode, it is possible to suppressthe electrons penetrating the light-emitting layer 214 and reaching thehole-transporting layer 212. Therefore, deterioration of thehole-transporting layer 212, which is caused by electrons reaching thehole-transporting layer 212, can be suppressed. Accordingly,deterioration of the light-emitting element can be prevented, and thelight-emitting element with a long lifetime can be obtained.

In addition, it is preferable that the emission colors of thelight-emitting layer and the fourth organic compound be similar colorsin this Embodiment Mode. For example, when the organic compoundcontained in the light-emitting layer is an organic compound whichexhibits bluish light emission such as YGA2S or YGAPA, the fourthorganic compound is preferably a substance which exhibits blue to bluishgreen light emission such as acridone, coumarin 102, coumarin 6H,coumarin 480D, or coumarin 30. Thus, even if the fourth organic compoundemits light unintentionally, the color purity of light emitted from thelight-emitting element can be maintained.

Further, when the organic compound contained in the light-emitting layeris an organic compound which exhibits greenish light emission such as2PCAPA, 2PCABPhA, 2DPAPA, 2DPABPhA, 2YGABPhA, or DPhAPhA, the fourthorganic compound is preferably a substance which exhibits bluish greento yellowish green light emission, such as N,N′-dimethylquinacridone(abbreviation: DMQd), N,N′-diphenylquinacridone (abbreviation: DPQd),9,18-dihydrobenzo[h]benzo[7,8]quino[2,3-b]acridine-7,16-dione(abbreviation: DMNQd-1),9,18-dihydro-9,18-dihydrobenzo[h]benzo[7,8]quino[2,3-b]acridine-7,16-dione(abbreviation: DMNQd-2), cumarin 30, cumarin 6, cumarin 545T, or cumarin153.

Further, when the organic compound contained in the light-emitting layeris an organic compound which exhibits yellowish light emission such asrubrene or BPT, the fourth organic compound is preferably a substancewhich exhibits yellowish green to yellowish orange light emission, suchas DMQd or(2-{2-[4-(9H-carbazol-9-yl)phenyl]ethenyl}-6-methyl-4H-pyran-4-ylidene)propanedinitrile(abbreviation: DCMCz).

Further, when the organic compound contained in the light-emitting layeris an organic compound which exhibits reddish light emission, such asp-mPhTD or p-mPhAFD, the fourth organic compound is preferably asubstance which exhibits orange to red light emission, such as2-{2-[4-(dimethylamino)phenyl]ethenyl}-6-methyl-4H-pyran-4-ylidene)propanedinitrile(abbreviation: DCM1),{2-methyl-6-[2-(2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile(abbreviation: DCM2),{2-(1,1-dimethylethyl)-6-[2-(2,3,6,7-tetrahydro-1,1,7,7-tetramethyl-1H,5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile(abbreviation: DCJTB), or Nile red.

Further, when the light-emitting material contained in thelight-emitting layer is a phosphorescent compound, the fourth organiccompound is also preferably a phosphorescent compound. For example, whenthe light-emitting material is the above-mentioned Ir(btp)₂(acac) whichexhibits red light emission, the fourth organic compound may be aphosphorescent compound which exhibits red light emission, such as(acetylacetonato)bis[2,3-bis(4-fluorophenyl)quinoxalinato]iridium(III)(abbreviation: Ir(Fdpq)₂(acac)). Note that the above-mentioned compoundsare compounds having a low LUMO level among compounds that can be usedfor light-emitting elements. Thus, by adding such a compound to thethird organic compound which will be described later, an excellentelectron-trapping property can be obtained.

For the fourth organic compound, among the above substances which arelisted above, a quinacridone derivative such as DMQd, DPQd, DMNQd-1, orDMNQd-2 is chemically stable and thus preferable. That is, by applying aquinacridone derivative, the lifetime of the light-emitting element canbe especially longer. Further, since quinacridone derivatives exhibitgreenish light emission, the element structure of the light-emittingelement described in this embodiment mode is especially effective for alight-emitting element exhibiting greenish light emission. Since a greencolor requires the highest level of luminance in forming a full-colordisplay, a green light-emitting element may be more deteriorated thanother light-emitting elements. However, such a problem can be overcomeaccording to the present invention.

Note that the fourth organic compound is preferably a coumarinderivative such as coumarin 102, coumarin 6H, coumarin 480D, coumarin30, coumarin 6, coumarin 545T, or coumarin 153. A coumarin derivativehas a low electron-trapping property. Therefore, the concentrationthereof added to the third organic compound may be relatively high. Thatis, the concentration can easily be controlled, and thus, a layer forcontrolling the carrier transport which has a desired property can beobtained. Further, since a coumarin derivative has high light emissionefficiency, decrease in light emission efficiency of the entirelight-emitting element can be suppressed when the fourth organiccompound emits light.

The third organic compound contained in the layer 215 for controllingthe electron transport is an organic compound having anelectron-transporting property. That is, the third organic compound is asubstance whose electron-transporting property is higher than thehole-transporting property. Specifically, any of the following can beused: a metal complex such as Alq, Almq₃, BeBq₂, BAlq, Znq, BAlq, ZnPBO,or ZnBTZ; a heterocyclic compound such as PBD, OXD-7, TAZ, TPBI, BPhen,or BCP; and a condensed aromatic compound such as CzPA, DPCzPA, DPPA,DNA, t-BuDNA, BANT, DPNS, DPNS2, or TPB3.

Further, a high-molecular compound such aspoly[(9,9-dihexylfluorene-2,7-diyl)-co-(pyridine-3,5-diyl)](abbreviation: PF-Py), orpoly[(9,9-dioctyllfluorene-2,7-diyl)-co-(2,2′-bipyridine-6,6′-diyl)](abbreviation: PF-BPy) can be used. Among them, a metal complex which isstable against electrons is preferably used.

As described above, the LUMO level of the fourth organic compound ispreferably lower than that of the third organic compound by 0.3 eV ormore. Therefore, a substance for the third organic compound may beselected as appropriate so as to satisfy such a condition, according tothe kind of the fourth organic compound which is used. For example, whenDPQd or coumarin 6 is used as the fourth organic compound, the abovecondition can be satisfied by using Alq as the third organic compound.

In addition, it is preferable that the emission color of the fourthorganic compound contained in the layer 215 for controlling the electrontransport and the emission color of the substance having a highlight-emitting property contained in the light-emitting layer 214 besimilar colors. Specifically, it is preferable that the differencebetween the wavelength of the highest peak of the emission spectrum ofthe fourth organic compound and the wavelength of the highest peak ofthe emission spectrum of the substance having a high light-emittingproperty be 30 nm or less. When the difference between the wavelengthsof the highest peaks is 30 nm or less, the emission color of the fourthorganic compound and the emission color of the substance having a highlight-emitting property can be similar colors. Accordingly, even in thecase where the fourth organic compound emits light due to change involtage or the like, change in emission color of the light-emittingelement can be suppressed.

The fourth organic compound does not always need to emit light. Forexample, in the case where light emission efficiency of the substancehaving a high light-emitting property is higher than that of the fourthorganic compound, the concentration of the fourth organic compound inthe layer 215 for controlling the electron transport is preferablyadjusted (the concentration is slightly lowered so that light emissionfrom the fourth organic compound is suppressed) so that light emittedonly from the substance having a high light-emitting property issubstantially obtained. In this case, since the emission colors of thesubstance having a high light-emitting property and the fourth organiccompound are similar colors (that is, the substance having a highlight-emitting property and the fourth organic compound havesubstantially the same energy gap), energy transfer from the substancehaving a high light-emitting property to the fourth organic compounddoes not easily occur, and thus, high light emission efficiency isobtained.

In addition, the thickness of the layer 215 for controlling the electrontransport is preferably from 5 nm to 20 nm, inclusive. When thethickness of the layer 215 for controlling the electron transport is toolarge, the carrier transporting rate becomes too slow, which couldresult in high driving voltage. When the thickness of the layer 215 forcontrolling the electron transport is too small, on the other hand, itis impossible to implement the function of controlling the carriertransport. Therefore, the thickness is preferably from 5 nm to 20 nm,inclusive.

The electron-transporting layer 216 is a layer containing a substancehaving a high electron-transporting property. For example, a metalcomplex such as tris(8-quinolinolato)aluminum(III) (abbreviation: Alq),tris(4-methyl-8-quinolinolato)aluminum(III) (abbreviation: Almq₃),bis(10-hydroxybenzo[h]quinolinato)beryllium(II) (abbreviation: BeBq₂),bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum(III)(abbreviation: BAlq), bis(8-quinolinolato)zinc(II) (abbreviation: Znq),bis[2-(2-benzoxazolyl)phenolato]zinc(II) (abbreviation: ZnPBO), orbis[2-(2-benzothiazolyl)phenolato]zinc(II) (abbreviation: Zn(BTZ)₂),which is a low molecular organic compound, can be used. Further, as analternative to such a metal complex, a heterocyclic compound such as2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation:PBD), 1,3-bis[5-p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene(abbreviation: OXD-7),3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole(abbreviation: TAZ01),2,2′,2″-(1,3,5-benzenetriyl)tris(1-phenyl-1H-benzimidazole)(abbreviation: TPBI), bathophenanthroline (abbreviation: BPhen), orbathocuproine (abbreviation: BCP) can be used. Most of the substancesmentioned here have an electron mobility of 10⁻⁶ cm²/Vs or higher. Notethat another substance may be used for the electron transporting layeras long as the electron-transporting property thereof is higher than thehole-transporting property. Note that the electron transporting layer isnot limited to a single layer, and may be two or more stacked layerscontaining any of the above-mentioned substances.

Further, for the electron-transporting layer 216, a high molecularcompound can be used. For example,poly[(9,9-dihexylfluorene-2,7-diyl)-co-(pyridine-3,5-diyl)](abbreviation: PF-Py),poly[(9,9-dioctyllfluorene-2,7-diyl)-co-(2,2′-bipyridine-6,6′-diyl)](abbreviation: PF-BPy), or the like can be used.

The electron-injecting layer 217 is a layer containing a substancehaving a high electron-injecting property. As a substance having a highelectron-injecting property, an alkali metal, an alkaline earth metal,or a compound thereof such as lithium fluoride (LiF), cesium fluoride(CsF), or calcium fluoride (CaF₂) can be used. For example, a layerformed of a substance having an electron-transporting property whichfurther contains an alkali metal, an alkaline earth metal, or a compoundthereof; for example, a layer of Alq containing magnesium (Mg) can beused. Note that by applying the aforementioned layer to theelectron-injecting layer 217, electrons can be efficiently injected fromthe second electrode 204, which is preferable.

As a substance for forming the second electrode 204, a metal, an alloy,an electrically conductive compound, or a mixture thereof, or the likewith a low work function (specifically, a work function of 3.8 eV orlower is preferable) can be used. Specific examples of such cathodematerials are given below: elements belonging to Group 1 and Group 2 ofthe periodic table, that is, alkali metals such as lithium (Li) andcesium (Cs) and alkaline earth metals such as magnesium (Mg), calcium(Ca), and strontium (Sr); alloys thereof (e.g., MgAg and AlLi); rareearth metals such as europium (Eu) and ytterbium (Yb); alloys thereof;and the like. A film of an alkali metal, an alkaline earth metal, or analloy thereof can be formed by a vacuum evaporation method. In addition,a film of an alloy including an alkali metal or an alkaline earth metalcan be formed by a sputtering method. Further, a film can be formedusing a silver paste or the like by an inkjet method or the like.

By providing the electron-injecting layer 217 which is a layer having afunction of promoting electron injection between the second electrode204 and the electron-transporting layer 216, the second electrode 204can be formed using various conductive materials such as Al, Ag, ITO, orindium tin oxide containing silicon or silicon oxide, regardless oftheir work functions. A film of such a conductive material can be formedby a sputtering method, an inkjet method, a spin coating method, or thelike.

Various methods can be used for forming an EL layer, regardless ofwhether the method is a dry process or a wet process. For example, avacuum evaporation method, an inkjet method, a spin coating method, orthe like may be used. Further, a different film formation method may beused to form each electrode or each layer.

For example, the EL layer may be formed by a wet process using a highmolecular compound selected from the above-mentioned materials. Further,the EL layer can also be formed by a wet process using a low molecularorganic compound. Further, the EL layer may be formed by a dry processsuch as a vacuum evaporation method using a low molecular organiccompound.

The electrode may also be formed by a wet process using a sol-gelmethod, or by a wet process using a paste of a metal material. Further,the electrode may be formed by a dry method such as a sputtering methodor a vacuum evaporation method.

For example, in a case where a light-emitting element of the presentinvention is applied to a display device and a light-emitting layer foreach color is formed separately, the light-emitting layer is preferablyformed by a wet process. Formation of the light-emitting layer by aninkjet method facilitates separate formation of the light-emitting layerfor each color even when a large-sized substrate is used.

In a light-emitting element of the present invention having theabove-described structure, current flows due to a potential differencegenerated between the first electrode 202 and the second electrode 204,whereby holes and electrons are recombined in the EL layer 203 and lightis emitted.

Light emission is extracted to the outside through one of or both thefirst electrode 202 and the second electrode 204. Accordingly, one of orboth the first electrode 202 and the second electrode 204 are electrodeshaving a light-transmitting property. When only the first electrode 202has a light-transmitting property, light emission is extracted from thesubstrate side through the first electrode 202 as illustrated in FIG.3A. When only the second electrode 204 has a light-transmittingproperty, light emission is extracted from the opposite side to thesubstrate through the second electrode 204 as illustrated in FIG. 3B.When both the first electrode 202 and the second electrode 204 have alight-transmitting property, light emission is extracted from both thesubstrate side and the opposite side to the substrate through the firstelectrode 202 and the second electrode 204 as illustrated in FIG. 3C.

Note that the structure of the layers provided between the firstelectrode 202 and the second electrode 204 is not limited to the abovestructure. Any structure other than the above structure can be employedas long as a layer for controlling the carrier transport is provided anda light-emitting region where holes and electrons are recombined ispositioned away from the first electrode 202 and the second electrode204 so as to prevent quenching caused by proximity of the light-emittingregion to metal.

That is, there is no particular limitation on the stacked structure ofthe layers, and a layer for controlling the carrier transport and thelight-emitting layer in this embodiment mode may be combined with alayer formed of a substance having a high electron-transportingproperty, a substance having a high hole-transporting property, asubstance having a high electron-injecting property, a substance havinga high hole-injecting property, a bipolar substance (a substance havinghigh electron-transporting and hole-transporting properties), or thelike.

The layer for controlling the hole transport controls the holetransport, and therefore, is preferably provided between thelight-emitting layer and the electrode serving as an anode.

As illustrated in FIGS. 1A and 1C, in a case of a structure where thelight-emitting layer 214 and the layer 213 for controlling the holetransport are in contact with each other, as the second organiccompound, it is preferable to use an organic compound into whichelectrons are not easily injected and the band gap thereof is higherthan that of an organic compound which has the highest weight percent inthe light-emitting layer 214. In a case where the light-emitting layer214 and the layer 213 for controlling the hole transport are in contactwith each other, the light-emitting layer and the layer for controllingthe carrier transport can be successively formed with the same mask,which is preferable in manufacturing a full-color display or the likewhere selective formation of the layer for controlling the carriertransport is needed for each light-emitting element because themanufacture is facilitated.

Note that as illustrated in FIGS. 1B and 1D, a structure in which alayer is formed between the light-emitting layer 214 and the layer 213for controlling the hole transport may be employed.

The layer for controlling the electron transport controls the electrontransport, and therefore, is preferably provided between thelight-emitting layer and the electrode serving as a cathode. Asillustrated in FIGS. 1A and 1B, a layer 215 for controlling the electrontransport is more preferably provided to be in contact with thelight-emitting layer 214. By providing the layer for controlling theelectron transport to be in contact with the light-emitting layer,electron injection into the light-emitting layer can be directlycontrolled. Therefore, change in carrier balance with time in thelight-emitting layer can be controlled more efficiently, whereby thelifetime of the element can be more effectively improved. In addition,the process can be simplified.

Note that the layer for controlling the electron transport is preferablyprovided to be in contact with the light-emitting layer, and in such acase, it is preferable that the third organic compound contained in thelayer for controlling the electron transport be a different kind of anorganic compound from an organic compound which has a high weightpercent in the light-emitting layer. In particular, in a case where thelight-emitting layer contains a substance (a fifth organic compound) fordispersing a substance having a high light-emitting property and asubstance having a high light-emitting property (a sixth organiccompound), the kinds of the fifth organic compound and the third organiccompound are preferably different from each other. In such a structure,the electron transport from the layer for controlling the electrontransport to the light-emitting layer is suppressed also between thethird organic compound and the fifth organic compound, and thus effectof providing the layer for controlling the electron transport is furtherincreased.

As illustrated in FIGS. 1C and 1D, a layer may be formed between thelight-emitting layer 214 and the layer 215 for controlling the electrontransport.

In addition, as in FIGS. 2A to 2D, a structure in which the secondelectrode 204 serving as a cathode, the EL layer 203, and the firstelectrode 202 serving as an anode are stacked in that order over thesubstrate 201 may be employed. The light-emitting element in FIG. 2A hasa structure in which the layers of the EL layer in FIG. 1A are stackedin the reverse order, the light-emitting element in FIG. 2B has astructure in which the layers of the EL layer in FIG. 1B are stacked inthe reverse order, the light-emitting element in FIG. 2C has a structurein which the layers of the EL layer in FIG. 1C are stacked in thereverse order, and the light-emitting element in FIG. 2D has a structurein which the layers of the EL layer in FIG. 1D are stacked in thereverse order.

Note that in this embodiment mode, the light-emitting element is formedover a substrate made of glass, plastic, or the like. By forming aplurality of such light-emitting elements over a substrate, a passivematrix light-emitting device can be manufactured. Further, for example,thin film transistors (TFTs) may be formed over a substrate made ofglass, plastic, or the like so that light-emitting elements aremanufactured over electrodes which are electrically connected to theTFTs. Thus, an active matrix light-emitting device which controls thedriving of a light-emitting element by a TFT can be manufactured. Notethat a structure of the TFT is not particularly limited, and either astaggered TFT or an inverted staggered TFT may be used. In addition, adriving circuit formed over a TFT substrate may be formed using anN-channel TFT and a P-channel TFT, or may be formed using either anN-channel TFT or a P-channel TFT. In addition, the crystallinity of asemiconductor film used for the TFT is not particularly limited. Eitheran amorphous semiconductor film or a crystalline semiconductor film maybe used for the TFT. Further, a single crystal semiconductor film may beused. A single crystal semiconductor film can be formed by a Smart Cut(registered trademark) method or the like.

As described above, the light-emitting element in this embodiment modeis characterized by having both the layer 213 for controlling the holetransport and the layer 215 for controlling the electron transport.

For example, in a conventional light-emitting element where the layer213 for controlling the hole transport and the layer 215 for controllingthe electron transport are not provided, holes injected from the firstelectrode 202 pass through the hole-injecting layer 211 and thehole-transporting layer 212 to be injected into the light-emitting layer214 without the transport being retarded; therefore, some of the holesreach the vicinity of the interface between the light-emitting layer andthe electron-transporting layer 216. If holes reach theelectron-transporting layer 216, the electron-transporting layer 216 maybe deteriorated. If the number of holes which reach theelectron-transporting layer 216 is increased with time due to thedeterioration, the recombination probability in the light-emitting layer214 is reduced with time, which results in reduction in element lifetime(luminance decay with time). Similarly, electrons injected from thesecond electrode 204 pass through the electron-injecting layer 217 andthe electron-transporting layer 216 to be injected into thelight-emitting layer 214 without the transport being retarded;therefore, some of the electrons reach the vicinity of the interfacebetween the hole-transporting layer 212 and the light-emitting layer214. If electrons reach the hole-transporting layer 212, thehole-transporting layer 212 may be deteriorated. If the number ofelectrons which reach the hole-transporting layer 212 is increased withtime due to the deterioration, the recombination probability in thelight-emitting layer 214 is reduced with time, which results inreduction in element lifetime (luminance decay with time).

On the other hand, as for a light-emitting element of the presentinvention, by providing the layer 213 for controlling the holetransport, holes injected from the first electrode 202 pass through thehole-injecting layer 211 and the hole-transporting layer 212 to beinjected into the layer 213 for controlling the hole transport. Thetransport of the holes injected into the layer 213 for controlling thehole transport is retarded, and hole injection into the light-emittinglayer 214 is controlled. As a result, holes become less likely to reachthe electron-transporting layer 216 and deteriorate theelectron-transporting layer 216. Note that it is important in thepresent invention that an organic compound which reduces ahole-transporting property is added to an organic compound having ahole-transporting property, instead of just applying a substance withlow hole mobility in the layer 213 for controlling the hole transport.With such a structure, in addition to just controlling hole injectioninto the light-emitting layer, change in the quantity of controlled holeinjection with time can be suppressed.

Further, in a light-emitting element of the present invention, the layer215 for controlling the electron transport is also provided. Thus,electrons injected from the second electrode 204 pass through theelectron-injecting layer 217 and the electron-transporting layer 216 tobe injected into the layer 215 for controlling the electron transport.Here, the layer 215 for controlling the electron transport has astructure in which the fourth organic compound having ahole-transporting property is added to the third organic compound havingan electron-transporting property. Therefore, the transport of theelectrons injected into the layer 215 for controlling the electrontransport is retarded, and electron injection into the light-emittinglayer 214 is controlled. As a result, electrons become less likely toreach the hole-transporting layer 212 and deteriorate thehole-transporting layer 212. Similarly, as for holes, holes becomefurther less likely to reach and deteriorate the electron-transportinglayer 216 because the layer 215 for controlling the electron transportincludes the third organic compound having an electron-transportingproperty. Note that it is important in the present invention that anorganic compound which reduces an electron-transporting property isadded to an organic compound having an electron-transporting property,instead of just applying a substance with low electron mobility in thelayer 215 for controlling the electron transport. With such a structure,in addition to just controlling electron injection into thelight-emitting layer 214, change in the quantity of controlled electroninjection with time can be suppressed.

Therefore, by controlling the quantity of injection of both carriers ofholes and electrons into the light-emitting layer, a light-emittingelement according to the present invention can prevent a phenomenon thatcarrier balance is lost and recombination probability is reduced withtime. Thus, the lifetime of the element can be improved (luminance decaywith time can be suppressed).

Further, as an effect of the layer 213 for controlling the holetransport, improvement in light emission efficiency can be given. In aconventional element where the layer 213 for controlling the holetransport is not provided, most of the holes injected from the firstelectrode 202 are injected into the light-emitting layer 214 without thetransport being controlled. If the light-emitting layer 214 has anelectron-transporting property, that is, if the material which has thehighest weight percent in the light-emitting layer 214 has anelectron-transporting property, a light-emitting region is formed in thevicinity of the interface between the light-emitting layer 214 and thehole-transporting layer 212. In addition, there is a possibility thatcations are generated by excessive holes in the vicinity of theinterface between the light-emitting layer 214 and the hole-transportinglayer 212. Since a cation serves as a quencher, light emissionefficiency decreases due to cations generated in the vicinity of thelight-emitting region.

However, by providing the layer 213 for controlling the hole transportdescribed in this embodiment mode, formation of cations generated byexcessive holes in the light-emitting layer 214 and in the vicinity ofthe light-emitting layer 214 can be suppressed, and decrease in lightemission efficiency can be suppressed. Accordingly, a light-emittingelement with high light emission efficiency can be obtained.

As described above, the light-emitting element in this embodiment modehas a layer for controlling the carrier transport. Since the layer forcontrolling the carrier transport contains two or more kinds ofsubstances, carrier balance can be controlled precisely by control ofcombination, the mixture ratio, the film thickness, or the like of thesubstances.

Further, since the carrier balance can be controlled by controllingcombination, the mixture ratio, the film thickness, or the like of thesubstances, control of the carrier balance can be easier than aconventional light-emitting element. That is, even if a physicalproperty of the substance itself is not changed, the carrier transportcan be controlled by controlling the mixture ratio, the film thickness,or the like.

Among two or more kinds of substances contained in the layer forcontrolling the carrier transport, an organic compound which has a lowerweight percent than another substance is used for controlling thecarrier transport. That is, the carrier transport can be controlled by acomponent which has a lower weight percent than another componentcontained in the layer for controlling the carrier transport. Therefore,a light-emitting element with a long lifetime, which does not easilydeteriorate with time, can be realized. In other words, carrier balancehardly changes as compared with the case where the carrier balance iscontrolled by a single substance. For example, if the carrier transportis controlled by a layer formed of a single substance, the balance ofthe whole layer is changed by partial change in morphology, partialcrystallization, or the like; therefore, the layer easily deteriorateswith time. However, as described in this embodiment mode, by controllingthe carrier transport by a component which has a lower weight percentthan another component contained in the layer for controlling thecarrier transport, an effect of change in morphology, crystallization,aggregation, or the like is reduced, and thus, deterioration with timeis hardly caused. Thus, a light-emitting element with a long lifetimecan be obtained in which light emission efficiency hardly decreases withtime.

Moreover, by controlling the carrier transport at opposite sides of thelight-emitting layer, an effect of change in morphology,crystallization, aggregation, or the like is further reduced, and thus,a light-emitting element with a long lifetime can be obtained in whichlight emission efficiency hardly decreases with time in whichdeterioration with time is hardly caused and light emission efficiencyhardly decreases with time can be obtained.

In addition, by controlling the carrier transport at opposite sides ofthe light-emitting layer, a light-emitting element with a long lifetimecan be obtained without depending on a carrier-transporting property ofthe light-emitting layer. Therefore, a material for the light-emittinglayer can be chosen from a wider range, and the light-emitting elementcan be designed more flexibly.

Note that this embodiment mode can be combined with another embodimentmode as appropriate.

Embodiment Mode 2

In this embodiment mode, a mode of a light-emitting element in which aplurality of light-emitting units according to the present invention arestacked (hereinafter this light-emitting element is referred to as astacked-type light-emitting element) will be described with reference toFIG. 6. This light-emitting element is a stacked-type light-emittingelement including a plurality of light-emitting units between a firstelectrode and a second electrode. Each structure of the light-emittingunits can be similar to the structure described in Embodiment Mode 1. Inother words, the light-emitting element described in Embodiment Mode 1is a light-emitting element having one light-emitting unit. In thisembodiment mode, a light-emitting element having a plurality oflight-emitting units will be described.

In FIG. 6, a first light-emitting unit 511 and a second light-emittingunit 512 are stacked between a first electrode 501 and a secondelectrode 502. As the first electrode 501 and the second electrode 502,similar electrodes to the electrode described in Embodiment Mode 1 canbe employed. Note that the first light-emitting unit 511 and the secondlight-emitting unit 512 may have the same structure or differentstructures, and as the structures, a similar structure to the structurein Embodiment Mode 1 can be employed.

A charge generation layer 513 contains a composite material in which asubstance having an acceptor property is mixed into an organic compound.This composite material of an organic compound and a substance having anacceptor property is the composite material described in Embodiment Mode1 and contains 7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane(abbreviation: F₄-TCNQ), or metal oxide such as vanadium oxide,molybdenum oxide, or tungsten oxide as a substance having an acceptorproperty. As an organic compound, any of various compounds such as anaromatic amine compound, a carbazole derivative, an aromatichydrocarbon, a high molecular compound, an oligomer, a dendrimer, or apolymer can be used. Note that the organic compound having a holemobility of 10⁻⁶ cm²/Vs or higher is preferably employed as an organiccompound. However, another substance may also be used as long as thehole-transporting property thereof is higher than theelectron-transporting property. A composite of an organic compound andmetal oxide is superior in a carrier-injecting property and acarrier-transporting property, and accordingly, low-voltage driving andlow-current driving can be realized

Note that the charge generation layer 513 may be formed with acombination of the composite material of an organic compound and asubstance having an acceptor property, and another material. Forexample, the charge generation layer 513 may be formed with acombination of a layer containing the composite material of an organiccompound and metal oxide, and a layer containing one compound selectedfrom substances having an electron-donating property and a compoundhaving a high electron-transporting property. Further, the chargegeneration layer 513 may be formed with a combination of a layercontaining the composite material of an organic compound and metaloxide, and a transparent conductive film. Furthermore, electrodematerials described in Embodiment Mode 1 can be used for the chargegeneration layer. Note that a layer having a high light-transmittingproperty is preferably used as the charge generation layer in terms oflight extraction efficiency.

In any case, the charge generation layer 513 interposed between thefirst light-emitting unit 511 and the second light-emitting unit 512 isacceptable as long as electrons are injected into a light-emitting uniton one side and holes are injected into a light-emitting unit on theother side when voltage is applied to the first electrode 501 and thesecond electrode 502. For example, in the case of applying voltage sothat potential of the first electrode is higher than potential of thesecond electrode, any structure is acceptable for the charge generationlayer 513 as long as the charge generation layer 513 injects electronsand holes into the first light-emitting unit 511 and the secondlight-emitting unit 512, respectively.

Although the light-emitting element having two light-emitting units isdescribed in this embodiment mode, a light-emitting element in whichthree or more light-emitting units are stacked can be employed in asimilar way. Like the light-emitting element of this embodiment mode, bydisposing a plurality of light emitting units between a pair ofelectrodes so as to be partitioned with the charge generation layer, theelement with a long lifetime in a high luminance region can be realizedwhile keeping low current density. In the case where the light-emittingelement is applied to lighting as an application example, voltage dropdue to resistance of an electrode material can be reduced. Accordingly,light can be uniformly emitted in a large area. Moreover, alight-emitting device of low power consumption, which can be driven atlow voltage, can be achieved.

The light-emitting units emit light of different colors from each other,thereby obtaining light emission of a desired color as the wholelight-emitting element. For example, in a light-emitting element havingtwo light-emitting units, by making the emission colors of the firstlight-emitting unit and the second light-emitting unit complementarycolors, the light-emitting element which emits white light as the wholeelement can be obtained. Note that complementary colors refer to colorswhich can produce an achromatic color when mixed. That is, white lightemission can be obtained by mixing light obtained from substancesemitting light of complementary colors. The same can be applied to alight-emitting element having three light-emitting units. For example,when the first light-emitting unit emits red light, the secondlight-emitting unit emits green light, and the third light-emitting unitemits blue light, white light can be emitted as the whole light-emittingelement.

Note that this embodiment mode can be combined with another embodimentmode as appropriate.

Embodiment Mode 3

In this embodiment mode, a light-emitting device having a light-emittingelement of the present invention will be described.

A light-emitting device having a light-emitting element of the presentinvention in a pixel portion is described in this embodiment mode withreference to FIGS. 7A and 7B. Note that FIG. 7A is a top viewillustrating the light-emitting device and FIG. 7B is a cross-sectionalview of FIG. 7A taken along lines A-A′ and B-B′. This light-emittingdevice includes a driver circuit portion (source side driver circuit)601, a pixel portion 602, and a driver circuit portion (gate side drivercircuit) 603, which are indicated by dotted lines, in order to controlthe light emission of the light-emitting element. Further, referencenumeral 604 indicates a sealing substrate and reference numeral 605indicates a sealing material. A space 607 is a portion surrounded by thesealing material 605.

Note that a leading wiring 608 is a wiring for transmitting signalsinput in the source side driver circuit 601 and the gate side drivercircuit 603. The leading wiring 608 receives video signals, clocksignals, start signals, reset signals and the like from a flexibleprinted circuit (FPC) 609 that serves as an external input terminal.Although only an FPC is illustrated here, this FPC may be provided witha printed wiring board (PWB). The light-emitting device in thisspecification refers to not only a light-emitting device itself but alsoto a state in which an FPC or a PWB is attached to a light-emittingdevice.

Then, a cross-sectional structure is described with reference to FIG.7B. While the driver circuit portion and the pixel portion are providedover an element substrate 610, FIG. 7B only illustrates the source sidedriver circuit 601, which is the driver circuit portion, and one pixelof the pixel portion 602.

Note that a CMOS circuit which is a combination of an N-channel TFT 623and a P-channel TFT 624 is provided in the source side driver circuit601. The driver circuit may be formed by various CMOS circuits, PMOScircuits, or NMOS circuits. In this embodiment mode, a driver-integratedtype in which a driver circuit is formed over a substrate is described;however, the present invention is not limited to this, and the drivercircuit can be formed outside the substrate.

The pixel portion 602 includes a plurality of pixels each having aswitching TFT 611, a current controlling TFT 612, and a first electrode613 that is electrically connected to a drain of the current controllingTFT 612. Note that an insulator 614 is formed to cover the edge of thefirst electrode 613. Here, a positive photosensitive acrylic resin filmis used to form the insulator 614.

Further, in order to improve the coverage, the insulator 614 is providedsuch that either an upper edge portion or a lower edge portion of theinsulator has a curved surface with a curvature. For example, whenpositive photosensitive acrylic is used as a material for the insulator614, it is preferable that only an upper edge portion of the insulator614 have a curved surface with a radius of curvature (0.2 μm to 3 μm).The insulator 614 can be formed using either a negative type thatbecomes insoluble in an etchant by light irradiation, or a positive typethat becomes dissoluble in an etchant by light irradiation.

An EL layer 616 and a second electrode 617 are formed over the firstelectrode 613. Here, various metals, alloys, electrically conductivecompounds, or mixtures thereof can be used for a material of the firstelectrode 613. If the first electrode is used as an anode, it ispreferable that the first electrode be formed using a metal, an alloy,an electrically conductive compound, a mixture thereof with a high workfunction (a work function of 4.0 eV or higher) among such materials. Forexample, the first electrode 613 can be formed using a single-layer filmsuch as an indium tin oxide film containing silicon, an indium zincoxide film, a titanium nitride film, a chromium film, a tungsten film, aZn film, a Pt film, or the like; a stacked film of a titanium nitridefilm and a film containing aluminum as its main component; or athree-layer structure of a titanium nitride film, a film containingaluminum as its main component, and a titanium nitride film. Note thatwhen a stacked structure is employed, the first electrode 613 has lowresistance as a wiring, forms a favorable ohmic contact, and can serveas an anode.

The EL layer 616 is formed by various methods such as an evaporationmethod using an evaporation mask, an inkjet method, a spin coatingmethod, or the like. The EL layer 616 includes the layer for controllingthe carrier transport described in Embodiment Mode 1 or Embodiment Mode2. Any of a low molecular compound, a high molecular compound, anoligomer, or a dendrimer may be employed as a material for the EL layer616. As the material for the EL layer, not only an organic compound butalso an inorganic compound may be used.

As the material for the second electrode 617, various types of metals,alloys, electrically conductive compounds, mixtures thereof, or the likecan be used. If the second electrode is used as a cathode, it ispreferable that the second electrode be formed using an alloy, anelectrically conductive compound, a mixture thereof with a high workfunction (a work function of 3.8 eV or higher) among such materials. Forexample, elements belonging to Group 1 and Group 2 of the periodictable, that is, alkali metals such as lithium (Li) and cesium (Cs) andalkaline earth metals such as magnesium (Mg), calcium (Ca), andstrontium (Sr); alloys thereof (e.g., MgAg and AlLi), and the like canbe given. In the case where light generated in the EL layer 616 istransmitted through the second electrode 617, the second electrode 617may also be formed by using a stacked layer of a thin metal film with areduced film thickness and a transparent conductive film (indium tinoxide (ITO), indium tin oxide containing silicon or silicon oxide,indium zinc oxide (IZO), indium oxide containing tungsten oxide and zincoxide (IWZO), or the like).

By attaching the sealing substrate 604 to the element substrate 610 withthe sealing material 605, the light-emitting element 618 is provided inthe space 607 which is surrounded by the element substrate 610, thesealing substrate 604, and the sealing material 605. Note that the space607 may be filled with a filler, and may be filled with an inert gas(such as nitrogen and argon), the sealing material 605, or the like.

As the sealing material 605, an epoxy-based resin is preferably used. Inaddition, it is desirable to use a material that allows permeation ofmoisture or oxygen as little as possible. As the sealing substrate 604,a plastic substrate formed of fiberglass-reinforced plastics (FRP),polyvinyl fluoride (PVF), polyester, acrylic, or the like can be used aswell as a glass substrate or a quartz substrate.

As described above, the light-emitting device having a light-emittingelement of the present invention can be obtained.

A light-emitting device of the present invention has the light-emittingelement described in Embodiment Mode 1 or Embodiment Mode 2. Thus, alight-emitting device with high light emission efficiency can beobtained.

In addition, since the light-emitting element with high light emissionefficiency is included, a light-emitting device with low powerconsumption can be obtained.

Further, since a light-emitting element with less deterioration and along lifetime is included, a light-emitting device with a long lifetimecan be obtained.

An active matrix light-emitting device that controls driving of alight-emitting element with a transistor is described above in thisembodiment mode; however, a passive matrix light-emitting device may beused. FIGS. 8A and 8B illustrate a passive matrix light-emitting devicemanufactured according to the present invention. Note that FIG. 8A is aperspective view of the light-emitting device and FIG. 8B is across-sectional view taken along a line X-Y in FIG. 8A. In FIGS. 8A and8B, an EL layer 955 is provided between an electrode 952 and anelectrode 956 over a substrate 951. The edge of the electrode 952 iscovered with an insulating layer 953. A partition layer 954 is providedover the insulating layer 953. The sidewalls of the partition layer 954slope so that the distance between one sidewall and the other sidewallis gradually reduced toward the surface of the substrate. In otherwords, the partition layer 954 is fabricated so that a lower side (aside in contact with the insulating layer 953) is shorter than an upperside (an opposite side of the side in contact with the insulating layer953) as illustrated in a cross-sectional view in FIG. 8B. The EL layer955 and the electrode 956 can be patterned by providing the partitionlayer 954 in this manner. In addition, in a passive matrixlight-emitting device, a light-emitting device with high light emissionefficiency can be obtained by including a light-emitting element withhigh light emission efficiency of the present invention.

A light-emitting device of the present invention has the light-emittingelement described in Embodiment Mode 1 or Embodiment Mode 2. Thus, alight-emitting device with high light emission efficiency can beobtained.

In addition, since the light-emitting element with high light emissionefficiency is included, a light-emitting device with low powerconsumption can be obtained.

Further, since a light-emitting element with less deterioration and along lifetime is included, a light-emitting device with a long lifetimecan be obtained.

Note that this embodiment mode can be combined with another embodimentmode as appropriate.

Embodiment Mode 4

In this embodiment mode, an electronic device of the present inventionwhich includes the light-emitting device described in Embodiment Mode 3will be described. An electronic device of the present invention has thelight-emitting element described in Embodiment Mode 1 or Embodiment Mode2, and thus has a display portion with high light emission efficiencyand low power consumption. In addition, the display portion has a longlifetime.

As an electronic device manufactured using a light-emitting device ofthe present invention, cameras such as a video camera and a digitalcamera, a goggle type display, a navigation system, an audio reproducingdevice (a car audio set, an audio component set, or the like), acomputer, a game machine, a portable information terminal (a mobilecomputer, a cellular phone, a portable game machine, an electronic bookreader, or the like), an image reproducing device provided with arecording medium (specifically, a device provided with a display devicethat can reproduce a recording medium and display the image such as adigital versatile disc (DVD)), and the like are given. Specific examplesof these electronic devices are illustrated in FIGS. 9A to 9D.

FIG. 9A illustrates a television device of this embodiment mode thatincludes a housing 9101, a support 9102, a display portion 9103, speakerportions 9104, a video input terminal 9105, and the like. In the displayportion 9103 of this television device, light-emitting elements similarto those described in Embodiment Modes 1 or Embodiment Mode 2 arearranged in matrix. One feature of the light-emitting element is thatlight emission efficiency is high and power consumption is low. Further,the light-emitting element has a long lifetime. The display portion 9103which includes the light-emitting element has similar features.Therefore, in this television device, image quality is hardlydeteriorated and low power consumption is achieved. With such features,deterioration compensation function and a power supply circuit can besignificantly reduced or downsized in the television device; therefore,reduction in size and weight of the housing 9101 and the support 9102can be achieved. In the television device of this embodiment mode, lowpower consumption, high image quality, and reduction in size and weightare achieved; therefore, a product which is suitable for livingenvironment can be provided.

FIG. 9B illustrates a computer of this embodiment mode that includes amain body 9201, a housing 9202, a display portion 9203, a keyboard 9204,an external connection port 9205, a pointing device 9206, and the like.In the display portion 9203 of this computer, light-emitting elementssimilar to those described in Embodiment Modes 1 or Embodiment Mode 2are arranged in matrix. One feature of the light-emitting element isthat light emission efficiency is high and power consumption is low.Further, the light-emitting element has a long lifetime. The displayportion 9203 which includes the light-emitting elements has similarfeatures. Therefore, in the computer, image quality is hardlydeteriorated and lower power consumption is achieved. With suchfeatures, deterioration compensation function and a power supply circuitcan be significantly reduced or downsized in the computer; therefore,reduction in size and weight of the main body 9201 and the housing 9202can be achieved. In the computer of this embodiment mode, low powerconsumption, high image quality, and reduction in size and weight areachieved; therefore, a product which is suitable for environment can beprovided. Moreover, the computer can be carried and the computer havingthe display portion which has strong resistance to external impact whenbeing carried can be provided.

FIG. 9C illustrates a cellular phone of this embodiment mode thatincludes a main body 9401, a housing 9402, a display portion 9403, anaudio input portion 9404, an audio output portion 9405, operation keys9406, an external connection port 9407, an antenna 9408, and the like.In the display portion 9403 of this cellular phone, light-emittingelements similar to those described in Embodiment Modes 1 or EmbodimentMode 2 are arranged in matrix. One feature of the light-emitting elementis that light emission efficiency is high and power consumption is low.Further, the light-emitting element has a long lifetime. The displayportion 9403 which includes the light-emitting elements has similarfeatures. Therefore, in the cellular phone, image quality is hardlydeteriorated and lower power consumption is achieved. With suchfeatures, deterioration compensation function and a power supply circuitcan be significantly reduced or downsized in the cellular phone;therefore, reduction in size and weight of the main body 9401 and thehousing 9402 can be achieved. In the cellular phone of this embodimentmode, low power consumption, high image quality, and reduction in sizeand weight are achieved; therefore, a product which is suitable forbeing carried can be provided. Further, the present invention canprovide a product, a display portion of which is resistant to impacteven when being carried.

FIG. 9D illustrates a camera that includes a main body 9501, a displayportion 9502, a housing 9503, an external connection port 9504, a remotecontrol receiving portion 9505, an image receiving portion 9506, abattery 9507, an audio input portion 9508, operation keys 9509, aneyepiece portion 9510, and the like. In the display portion 9502 of thiscamera, light-emitting elements similar to those described in EmbodimentModes 1 or Embodiment Mode 2 are arranged in matrix. One feature of thelight-emitting element is that light emission efficiency is high andpower consumption is low. Further, the light-emitting element has a longlifetime. The display portion 9502 which includes the light-emittingelements has similar features. Therefore, in the camera, image qualityis hardly deteriorated and lower power consumption is achieved. Withsuch features, deterioration compensation function and a power supplycircuit can be significantly reduced or downsized in the camera, therebyachieving reduction in size and weight of the main body 9501. In thecamera of this embodiment mode, low power consumption, high imagequality, and reduction in size and weight are achieved; therefore, aproduct which is suitable for being carried can be provided. Further,the present invention can provide a product, a display portion of whichis resistant to impact even when being carried.

FIG. 10 illustrates an audio reproducing device, specifically, a caraudio system. The audio reproducing device includes a main body 701, adisplay portion 702, and operation switches 703 and 704. The displayportion 702 can be realized by the light-emitting device (passive matrixor active matrix) described in Embodiment Mode 2. Further, the displayportion 702 may be formed using a segment type light-emitting device. Inany case, the use of a light-emitting element of the present inventionmakes it possible to form a bright display portion with a long lifetimewhile achieving low power consumption which uses a vehicle power source(12 to 42 V). Further, although this embodiment mode describes an in-caraudio system, a light-emitting device of the present invention may alsobe used in portable audio systems or audio systems for home use.

FIG. 11 illustrates a digital player as one example of that. The digitalplayer illustrated in FIG. 11 includes a main body 710, a displayportion 711, a memory portion 712, an operation portion 713, earphones714, and the like. Note that a pair of headphones or a pair of wirelessearphones can be used instead of the pair of earphones 714. The displayportion 711 can be realized by the light-emitting device (passive matrixor active matrix) described in Embodiment Mode 2. Further, the displayportion 711 may be formed using a segment type light-emitting device. Inany case, by using a light-emitting element of the present invention, adisplay portion can be formed that is capable of displaying images evenwhen using a secondary battery (a nickel-hydrogen battery or the like),has a long lifetime, is bright, and achieves low power consumption. Asthe memory portion 712, a hard disk or a nonvolatile memory is used. Forexample, a NAND type nonvolatile memory with a recording capacity of 20to 200 gigabytes (GB) is used, and by operating the operation portion713, an image or a sound (music) can be recorded and reproduced. Notethat in the display portion 711, white characters are displayed againsta black background, and thus, power consumption can be reduced. This isparticularly effective for portable audio systems.

As described above, the applicable range of the light-emitting devicemanufactured by applying the present invention is so wide that thelight-emitting device is applicable to electronic devices in variousfields. By applying the present invention, an electronic device whichhas a display portion consuming low power and having high reliabilitycan be manufactured.

A light-emitting device to which the present invention is applied has alight-emitting element with high light emission efficiency, and can alsobe used as a lighting device. One mode of using a light-emitting elementaccording to the present invention as a lighting device is describedwith reference to FIG. 12.

FIG. 12 illustrates an example of a liquid crystal display device usinga light-emitting device of the present invention as a backlight. Theliquid crystal display device illustrated in FIG. 12 includes a housing901, a liquid crystal layer 902, a backlight 903, and a housing 904. Theliquid crystal layer 902 is connected to a driver IC 905. Alight-emitting device of the present invention is used as the backlight903, and current is supplied through a terminal 906.

By using a light-emitting device of the present invention as a backlightof the liquid crystal display device, the backlight can have high lightemission efficiency. In addition, a backlight with a long lifetime canbe obtained. A light-emitting device of the present invention is a planeemission type lighting device, and can have a large area. Therefore, thebacklight can have a large area, and a liquid crystal display devicehaving a large area can be obtained. Furthermore, a light-emittingdevice of the present invention has a thin shape and consumes low power;therefore, thickness and power consumption of a display device can alsobe reduced.

FIG. 13 illustrates an example in which a light-emitting deviceaccording to the present invention is used as a desk lamp, which is oneof lighting devices. The desk lamp illustrated in FIG. 13 includes ahousing 2001 and a light source 2002, and a light-emitting device of thepresent invention is used as the light source 2002. Since alight-emitting device of the present invention has a long lifetime, thedesk lamp can also have a long lifetime.

FIG. 14 illustrates an example in which a light-emitting device to whichthe present invention is applied is used as an interior lighting device3001. Since a light-emitting device of the present invention can alsohave a large area, a light-emitting device of the present invention canbe used as a lighting device having a large emission area. Moreover,since a light-emitting device of the present invention has a longlifetime, the lighting device can also have a long lifetime. Atelevision device 3002 of the present invention like the television setin FIG. 9A may be placed in a room where a light-emitting deviceaccording to the present invention is used as the interior lightingdevice 3001, and public broadcasting or movies can be watched there. Insuch a case, since both of the devices have long lifetimes, frequency ofreplacement of the lighting device and the television device can bereduced, and environmental load can be reduced.

Note that this embodiment can be combined with another embodiment modeas appropriate.

This application is based on Japanese Patent Application Serial No.2007-243270 filed with Japan Patent Office on Sept. 20, 2007, the entirecontents of which are hereby incorporated by reference.

1. A light-emitting element comprising: a first electrode; a first layerformed over the first electrode, wherein the first layer comprises afirst organic compound and a second organic compound; a light-emittinglayer formed over the first layer; a second layer formed over thelight-emitting layer, wherein the second layer comprises a third organiccompound and a fourth organic compound; and a second electrode formedover the second layer, wherein a weight percent of the first organiccompound is higher than a weight percent of the second organic compound,wherein a weight percent of the third organic compound is higher than aweight percent of the fourth organic compound, wherein the first organiccompound has a hole transporting property, wherein the second organiccompound has an electron transporting property, wherein the thirdorganic compound has an electron transporting property, and wherein thefourth organic compound has an electron trapping property.
 2. Thelight-emitting element according to claim 1, wherein a differencebetween a highest occupied molecular orbital level of the first organiccompound and a highest occupied molecular orbital level of the secondorganic compound is less than 0.3 eV.
 3. The light-emitting elementaccording to claim 1, wherein the first organic compound is an aromaticamine compound, and wherein the second organic compound is a metalcomplex.
 4. The light-emitting element according to claim 1, whereinP₁/P₂≧3 or P₁/P₂≦0.33 is satisfied where P₁ is a dipole moment of thefirst organic compound and P₂ is a dipole moment of the second organiccompound.
 5. The light-emitting element according to claim 1, wherein athickness of the first layer is from 5 nm to 20 nm, inclusive, andwherein a thickness of the second layer is from 5 nm to 20 nm,inclusive.
 6. The light-emitting element according to claim 1, wherein alowest unoccupied molecular orbital level of the fourth organic compoundis lower than a lowest unoccupied molecular orbital level of the thirdorganic compound by 0.3 eV or more.
 7. The light-emitting elementaccording to claim 1, wherein the third organic compound is a metalcomplex.
 8. The light-emitting element according to claim 1, wherein thefourth organic compound is selected from the group consisting of aquinacridone derivative and a coumarin derivative.
 9. The light-emittingelement according to claim 1, wherein the first electrode is an anodeand has a light-transmitting property.
 10. The light-emitting elementaccording to claim 1, wherein the second electrode is a cathode and alight-transmitting property.
 11. The light-emitting element according toclaim 1, wherein the light-emitting layer comprises a fifth organiccompound and a sixth organic compound, and wherein the sixth organiccompound has a light-emitting property.
 12. The light-emitting elementaccording to claim 11, wherein the third organic compound and the fifthorganic compound are different from each other.
 13. An electronic devicecomprising, a display portion, wherein the display portion is providedwith a light-emitting element according to claim
 1. 14. A light-emittingelement comprising: a first electrode; a first layer formed over thefirst electrode; a second layer formed over the second layer, whereinthe second layer comprises a first organic compound and a second organiccompound; a light-emitting layer formed over the second layer; a thirdlayer formed over the light-emitting layer, wherein the third layercomprises a third organic compound and a fourth organic compound; afourth layer formed over the third layer; and a second electrode formedover the fourth layer, wherein a weight percent of the first organiccompound is higher than a weight percent of the second organic compoundin the second layer, wherein a weight percent of the third organiccompound is higher than a weight percent of the fourth organic compoundin the third layer, wherein the first layer has a hole injectingproperty, wherein the fourth layer has an electron injecting property,wherein the first organic compound has a hole transporting property,wherein the second organic compound has an electron transportingproperty, wherein the third organic compound has an electrontransporting property, and wherein the fourth organic compound has anelectron trapping property.
 15. The light-emitting element according toclaim 14, wherein a difference between a highest occupied molecularorbital level of the first organic compound and a highest occupiedmolecular orbital level of the second organic compound is less than 0.3eV.
 16. The light-emitting element according to claim 14, wherein thefirst organic compound is an aromatic amine compound, and wherein thesecond organic compound is a metal complex.
 17. The light-emittingelement according to claim 14, wherein P₁/P₂≧3 or P₁/P₂≦0.33 issatisfied where P₁ is a dipole moment of the first organic compound andP₂ is a dipole moment of the second organic compound.
 18. Thelight-emitting element according to claim 14, wherein a thickness of thesecond layer is from 5 nm to 20 nm, inclusive, and wherein a thicknessof the third layer is from 5 nm to 20 nm, inclusive.
 19. Thelight-emitting element according to claim 14, wherein a lowestunoccupied molecular orbital level of the fourth organic compound islower than a lowest unoccupied molecular orbital level of the thirdorganic compound by 0.3 eV or more.
 20. The light-emitting elementaccording to claim 14, wherein the third organic compound is a metalcomplex.
 21. The light-emitting element according to claim 14, whereinthe fourth organic compound is selected from the group consisting of acoumarin derivative and a quinacridone derivative.
 22. Thelight-emitting element according to claim 14, wherein the firstelectrode is an anode and has a light-transmitting property.
 23. Thelight-emitting element according to claim 14, wherein the secondelectrode is a cathode and has a light-transmitting property.
 24. Thelight-emitting element according to claim 14, wherein the light-emittinglayer comprises a fifth organic compound and a sixth organic compound,and wherein the sixth organic compound has a high light-emittingproperty.
 25. The light-emitting element according to claim 24, whereinthe third organic compound and the fifth organic compound are differentfrom each other.
 26. The light-emitting element according to claim 14,further comprising a fifth layer interposed between the first layer andthe second layer, wherein the fifth layer has a hole transportingproperty.
 27. The light-emitting element according to claim 14, furthercomprising a fifth layer interposed between the second layer and thelight-emitting layer, wherein the fifth layer has a hole transportingproperty.
 28. The light-emitting element according to claim 14, furthercomprising a sixth layer interposed between the third layer and thefourth layer, wherein the sixth layer has an electron transportingproperty.
 29. The light-emitting element according to claim 14, furthercomprising a sixth layer interposed between the light-emitting layer andthe third layer, wherein the sixth layer has an electron transportingproperty.
 30. An electronic device comprising, a display portion,wherein the display portion is provided with a light-emitting elementaccording to claim
 14. 31. A light-emitting element comprising: a firstelectrode; a second electrode; and a plurality of light-emitting unitsbetween the first electrode and the second electrode, wherein at leastone of the plurality of light-emitting units comprises: a first layercomprising a first organic compound and a second organic compound; asecond layer comprising a third organic compound and a fourth organiccompound; and a light-emitting layer interposed between the first layerand the second layer, wherein the first organic compound has a holetransporting property and the second organic compound has an electrontransporting property, and wherein the third organic compound has anelectron transporting property and the fourth organic compound has anelectron trapping property, and wherein the plurality of light-emittingunits are stacked.
 32. The light-emitting element according to claim 31,wherein emission colors of the plurality of light-emitting units aredifferent from each other.
 33. The light-emitting element according toclaim 32, wherein the light-emitting element emits white light.
 34. Thelight-emitting element according to claim 31, wherein a differencebetween a highest occupied molecular orbital level of the first organiccompound and a highest occupied molecular orbital level of the secondorganic compound is less than 0.3 eV.
 35. The light-emitting elementaccording to claim 31, wherein the first organic compound is an aromaticamine compound, and wherein the second organic compound is a metalcomplex.
 36. The light-emitting element according to claim 31, whereinP₁/P₂≧3 or P₁/P₂≦0.33 is satisfied where a dipole moment of the firstorganic compound is P₁ and a dipole moment of the second organiccompound is P₂.
 37. The light-emitting element according to claim 31,wherein a thickness of the first layer is from 5 nm to 20 nm, inclusive,and wherein a thickness of the second layer is from 5 nm to 20 nm,inclusive.
 38. The light-emitting element according to claim 31, whereina lowest unoccupied molecular orbital level of the fourth organiccompound is lower than a lowest unoccupied molecular orbital level ofthe third organic compound by 0.3 eV or more.
 39. The light-emittingelement according to claim 31, wherein the third organic compound is ametal complex.
 40. The light-emitting element according to claim 31,wherein the fourth organic compound is selected from the groupconsisting of a coumarin derivative and a quinacridone derivative. 41.The light-emitting element according to claim 31, wherein the firstelectrode has a light-transmitting property.
 42. The light-emittingelement according to claim 31, wherein the second electrode has alight-transmitting property.
 43. The light-emitting element according toclaim 31, wherein the light-emitting layer comprises a fifth organiccompound and a sixth organic compound, and wherein the sixth organiccompound has a high light-emitting property.
 44. The light-emittingelement according to claim 43, wherein the third organic compound andthe fifth organic compound are different from each other.
 45. Anelectronic device comprising, a display portion, wherein the displayportion is provided with a light-emitting element according to claim 31.